1. Field of the Invention
This invention relates generally to active sonar systems and more particularly to methods for optimizing noise-limaited and reverberation-limited target detection in littoral regions.
2. Description of the Prior Art
A major problem for sonar systems operating in shallow water is reverberation from the ocean bottom. With expanding Navy operation in littoral regions, the shallow-water reverberation. problem has received much recent attention from practitioners in the art. In 1995, Henry Cox et al. (xe2x80x9cGeometric Comb Waveformns for Reverberation Suppression,xe2x80x9d Proceedings, Twenty-Ninth Asilomar Conference on Signals, Systems, and Computers, Pacific Grove, Calif., Oct. 29-Nov. 1, 1995, pp. 1185-1189) proposed a class of geometric comb waveformns that offer high range resolution and, excellent Doppler properties for active sonar detection of moving targets in reverberation. In 1999, James Alsup (xe2x80x9cComb Waveforms for Sonar,xe2x80x9d Proceedings, Thirty-Third Asilomar Conference on Signals, Systems and Computers, Pacific Grove, Calif., Oct. 24-27, 1999, pp. 864-869) introduced the xe2x80x9ctriplet-pairxe2x80x9d waveform to achieve what the geometric comb waveform did with improved power efficiency. These waveforms maintain high Doppler sensitivity by using a plurality of narrow-band spectral components, for example, continuous-wave (CW) tone pulses, whose frequencies are placed so as to be almost, but not quite, equally-spaced within the system bandwidth.
Collins et al. (xe2x80x9cDoppler-Sensitive Active Sonar Pulse Designs for Reverberation Processing,xe2x80x9d IEEE Proc.-Radar, Sonar Navig., Vol. 145, No. 6, December 1998, pp. 347-353) later compared the theoretical and experimental performance of several reverberation-insensitive active sonar waveforms. Collins et al. showed that the hyperbolic frequency-modulated (HFM) waveform is advantageous for low Doppler targets at long ranges, because the system is usually noise-limited at long ranges, and that the sinusoidally frequency-modulated (SFM) pulse waveform is preferred for suppressing reverberation effects, which occur primarily at short to medium ranges. But they also noted that the Cox comb waveform eliminate much of the range-ambiguity of the SFM system while still exhibiting a similar Doppler ambiguity.
Some littoral regions have negligible reverberation and detection capability is accordingly ambient-noise limited over some portion of the nominal detection range of an active sonar system. This may occur in slightly deeper water at close range or in shallow water at longer range. Because active sonar transmitters suitable for littoral operation are normally power- and duty-cycle-limited, there is a need for transmit waveforms with a dynamic range designed to make use of as much available power as possible. Collins et al. suggest that the SFM waveform is preferred over the Cox comb waveform despite its superior range-ambiguity properties because of the improved noise-limited performance of the higher average transmitter power available from SFM. Alsup showed that the triplet-pair waveform has both the noise-limited performance of SFM and the superior range-ambiguity properties of the geometric comb, and thus (of these three) would be most preferred.
There are also many signal waveforms known in the art that offer a xe2x80x9cthumbtackxe2x80x9d type of ambiguity function (AF), the advantages of which may be appreciated with reference to C. E. Cook et al (Radar Signals, an Introduction to Theory and Practice, Academic Press, New York, 1967) or A. W. Rihaczek (Principles of High-Resolution Radar, McGraw-Hill, New York, 1969). These signals include, for example, pseudo-random noise (PRN) sequences, frequency-hop (FHOP) codes (spread-spectrum), and rooftop-FM signals. However, such signals do not provide adequate Doppler-ambiguity properties generally because of the integrated behavior of the sidelobes or the lack of sufficient chip-length.
There is accordingly still a clearly-felt need in the art for an active sonar system that provides improved performance in noise-limited littoral regions without sacrificing sensitivity to low-Doppler targets in reverberation-limited environments. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.
The system of this invention includes a new comb-like waveform constructed by modulating the tines of the comb according to a set of Hermite functions, and an improved processing method that reduces the sidelobes of the ambiguity function associated with the normally-processed comb-like waveform. Ambient noise-limited performance remains high, because the waveform is designed to be power-efficient; range ambiguity is superior to the highly-rated sinusoidal frequency-modulated (SFM) waveform; and reverberation-limited performance is equal to or better than that offered by any other waveform designed for this purpose, such as, for example, the geometric comb or triplet-pair comb waveforms.
It is a purpose of this invention to provide an active sonar system with optimum or near-optimum noise-limited performance in littoral regions, which normally occurs at longer ranges. It is another purpose of this invention to provide an active sonar system with optimum or near-optimum reverberation-limited performance in littoral regions, which normally occurs at short-to-medium range.
In one aspect, the invention is an acoustic detection method including the steps of transmitting an acoustic signal employing a Hermite function space (HFS) comb waveform to ensonify a target area, detecting acoustic reflections from the target area at a receiver transducer, generating a transducer output signal representing the acoustic reflections, and processing the transducer output signal to determine range and Doppler values for the target area.
In another aspect, the invention is an acoustic detection apparatus comprising an acoustic transmitter for transmitting an acoustic signal to ensonify a target area, wherein the acoustic signal includes a Hermite-function space (HFS) comb waveform, a receiver transducer array for detecting acoustic reflections from the target area, a circuit for generating a transducer array output signal representing the acoustic reflections, and a new type of signal processor for processing the transducer array output signals (including beamforming) to determine range, bearing and Doppler values for the target area.
The foregoing, together with other objects, features and advantages of this invention, can be better appreciated with reference to the following specification, claims and the accompanying drawing.